Hi,

I have to select a DC motor which can spin with maximum RPM of 3000 and minimum RPM of less than 200 , with given load of 200 gram Disc placed on it.

Now, how do I calculate the RPM needed with the load of 200 gram disc and the DC motor specification to select one.

Thanks in advance!

 

It depends on how fast you need to accelerate the disc.

Theoretically, the only real load would be bearing and aerodynamic losses if you don't care how long it takes to spin up/down.

 

Is this a school/college project?
Is the disc co-axial with the motor shaft?
Is the mass of the disc evenly distributed?

 

you will need to know the torque of motor

 

Is this a school/college project?
Is the disc co-axial with the motor shaft?
Is the mass of the disc evenly distributed?

Sorry for late reply,
the Disc is co-axial and the mass is eventually distributed,
I don't need a specific acceleration from 0 to particular speed, but once reached the set speed its rpm is constant.

I need to know how should I choose the DC motor in Market , such that it can hold disc of 200 grams ( 0.44 pounds ) with 100<RPM <3000.
It is needed that with 200 gram load, the motor is able to spin at minimum 100 rpm and max 3000 rpm.

Any help. Thanks.

 

Even the tiniest hobbyist model motor should be able to spin that load, if the load is perfectly balanced and the motor bearings are good ones.

 

Just guessing here, as I often do - but what about a scrapped motor out of a CD player? They spin fairly fast, but I don't know the specifics. They also run on low voltages like 3 to 5 volts. I've driven them with as much as 12 volts with no apparent harm to them; but I wouldn't recommend that for long periods of time.

Like I said, just a guess. One method for finding such is to grab things you can scrap out of other things, things like old printers for instance or a computer fan with the blades removed. Of course you'd have to balance the system, but those computer fans are often constructed with their magnets actually spinning around a brushless stator and some sort of IC to drive them. Perhaps if they operate on a specific frequency you could modify the frequency to something higher and see what happens. Hey! It's all junk stuff, so who cares what you blow up?!

 

Even the tiniest hobbyist model motor should be able to spin that load, if the load is perfectly balanced and the motor bearings are good ones.

But when I used the CD drive dc motor 5V , it was not able to spin below 800 rpm and not above 2200 rpm for load less than 50 grams. Why ?
Why was it stopped , if I go below 800 rpm ?
Thanks.

 

If the CD motor is a BLDC type it's commutation arrangement might limit the speed range.

 

But when I used the CD drive dc motor 5V , it was not able to spin below 800 rpm and not above 2200 rpm for load less than 50 grams. Why ?
Why was it stopped , if I go below 800 rpm ?

Honestly, I don't know the answer to this question. When I return home I'll take one apart. But that won't be before December this year. Alec_t may have the correct answer. Brushless DC motors have a series of coils that are switched on and off by a controlling chip of some sort. Also, likely they have a hall effect sensor that feeds back the RPM to that chip. In that case the chip and circuitry may be limiting the motor's capabilities between 800 and 2200 RPM. Again, I don't know the answer here. I've pulled computer fans apart and have learned what I've shared just from observation. I can conceive of the notion that the BLDC motor is regulated internally. If you try to govern speeds via voltage - that probably won't work. I have some older CD Drives to mess with. Maybe they're a little different internally. I'm sure someone here knows a better solution. Keep asking.

 

I don't think you can find or even expect a DC or any electric motor to vary its speed that much. Maybe if you post what you're trying to do it will get you better answers on how to do it.

 

I remember running my train set on a PWM supply. I was able to make the locomotive move realistically slow to (my estimates) a scale 70 MPH. (112.6 KPH for you metric people)

 

Put motor with disk in vacuum and it will easy rich desired 3000 rpm.
In my old experiments thin few gram cardboard 300mm diameter disk at 3000 RPM consumed 50W of mechanical power.
Because of friction with air.

 

I remember running my train set on a PWM supply. I was able to make the locomotive move realistically slow to (my estimates) a scale 70 MPH. (112.6 KPH for you metric people)

But that train engine also had gearing. The motor it's self ran much faster than the track speed. And the gearing also allowed it to crawl along at a very low speed.

 

Are you allowed to use feed back? In a project to motorize a Jag speedometer a Johnson DCM-471, 12V, 150 mA, US $ 4.50, spun from 1 mph to 130 mph or 5 RPM to 3000 RPM using a 20 slot disc & some electronics.

 

I never saw an answer as to if this was a school project or what. But a reasonable quality brush type DC motor with bearings in good condition should be be able to spin down to a very low speed as the voltage is reduced.The better the bearings the lower the speed that can be held.
But this still sounds like a school project, and like most of them there are additional requirements that we have not been told about.
Is this for a specific application? That makes a great deal of difference.

 

@shortbus Rain on my parade. Yeah, true, there was gearing involved. Still, even geared way the heck down - the motor still was able to crawl quite slowly. Since I don't know the frequency the PWM was at I couldn't say for sure the motor didn't receive full power at very low speeds for longer than 1KHz. The human eye can't detect frame rates lower than 28 frames per second. I'm wondering if a lower frequency PWM would have a closer affect on a motor to what the original post is asking for.

@MrBi11 I've been wondering the same thing. Sounds like an engineering homework assignment. We're going to engineer a solution and he's going to get the degree. Sad thing is that he will never be able to solve problems on his own unless he does the hard part. I'm studying for a CQI certificate. Lots of statistics and trig. Kicking my behind. But I'm getting it done.

@Aagash Is this school homework? How are you able to measure the RPMs of a motor? I'm not advanced enough to take such measurements, nor do I have the tools. If you're purely working with numbers then I could see how your numbers are working (sort of). I've usually just drilled a tiny hole in a soda bottle cap and stick it on the spindle of the motor. The ribbing on the side of the cap would rub against a small plastic tab and make a sound. I could hear the difference in motor speeds. But measure it? That'd require some sort of feedback. Maybe you're using BEMF to control motor speed, but again, how do you KNOW what RPM's you're seeing?

I'm thinking your problem is more hypothetical. Forgive me if I'm wrong, I'm not here to be an arse. I (and maybe "WE") need to know more about your project, it's purpose, your methods and tooling, among other things.

 

Measuring RPM without any contact is very straight-forward. A reflective spot on some rotating portion and a photoelectric pickup to detect the spot passing, coupled to drive a frequency counter. Multiply the frequency by 60 and you have RPM. In theory this could be done with a smartphone and a source of light. no other equipment needed. There are more sophisticated methods, but they all require a bit more equipment.